This reads like my life story. I actually went a little further - I came close to failing my undergrad degree because I was bored and partying too much, then I put in a ton of effort at the last minute - enough that one of my lecturers offered me the opportunity to do a PhD under him... then I started thinking about the futility of it all again, and slacked-off the first couple of years of the PhD!

Fortunately, the extreme ups-and-downs seem to be lessening as I get older.